A method to improve the physical properties of a polymer is the
addition of reinforcing particles or fibers to the polymer to form
composite materials. Owing to the development of high-strength and
light-weight fibers, such as carbon fiber and silicon carbide fiber, the
fiber-reinforced composites have been widely used. The recent
development of polymer nanocomposites containing nanometer scale
fillers, such as layered silicates or carbon nanotubes, provides a new
promising opportunity in composite materials [1-3].

Layered materials are well suited for the design of hybrid
composites, because their lamellar elements have high in-plane strength
and a high aspect ratio [4]. The smectite clays (e.g. montmorillonite)
and related layered silicates are the materials of choice for polymer
nanocomposite design, because they can not only be obtained easily from
nature at low cost but can also be modified chemically to make
themselves compatible with organic polymers. Each silicate layer has a
lateral dimension of 200-2000 nm and a thickness of about 1 nm. The
stacking of silicate layers forms tactoids that are typically 100-1000
nm thick. The cation-exchange property of smectite clays is an important
aspect of their use in nanocomposite formation [5].

The overall properties of a polymer/clay nanocomposite material are
determined not only by the parent components but also by the composite
phase morphology and interfacial properties. Nanocomposites usually
exhibit improved performances compared to conventional microcomposites,
owing to their unique phase morphology and improved interfacial
properties [4-9]. Major differences between conventional microcomposites
and nanocomposites result from the fact that the latter has much larger
surface (or interface) area per unit volume. Since many important
chemical and physical interactions are governed by surfaces, a
nanostructured composite can have substantially different properties
from a microstructured composite of the same composition [10, 11].

Two types of polymer/clay nanocomposite are possible. Intercalated
nanocomposites are formed when some organic molecules are inserted
between the silicate layers. Exfoliated nanocomposites are formed when
the silicate layers are individually dispersed in the polymer matrix.
Exfoliated nanocomposites show greater phase homogeneity than do
intercalated nanocomposites. This structural distinction is the primary
reason why the exfoliated structure is especially effective in improving
the reinforcement and other performances of polymer/clay nanocomposite
materials [5]. The ability of smectite clays to improve the mechanical
properties of an engineering plastic (Nylon 6) was first demonstrated by
Toyota researchers [12-14]. The key to this extraordinary performance of
Nylon 6/clay hybrids was the complete dispersal exfoliation of the
silicate layers in the polymer matrix.

[FIGURE 1 OMITTED]

The nanocomposite chemistry developed first for thermoplastics has
been extended in recent years to thermosets. The dimensional stability,
thermal stability, and solvent resistance of an epoxy resin, one of the
most common thermosets, can be improved when the silicate layers are
present [15-18]. Epoxy resins can be used in the adhesive, coating,
electronic, and aerospace industries, because they have excellent
thermo-mechanical and chemical properties [19]. The inherent excellent
properties of epoxy resins are a result of the curing process, in which
a low-molecular-weight resin is transformed into an infinite molecular
weight polymer with a three-dimensional network structure [20]. The
curing process of an epoxy resin would be affected by the incorporation
of a smectite clay. Therefore, in this study, an epoxy/clay
nanocomposite system has been prepared by mixing an organically modified
smectite clay with an epoxy resin, and their curing behavior has been
investigated to see the effect of the organoclay on the curing behavior
of the epoxy resin. Thermomechanical properties of the nanocomposite
system and the structure of the nanocomposite system were investigated,
to confirm the formation of intercalated and/or exfoliated nanostructure
in the epoxy/clay nanocomposite system.

EXPERIMENTAL

Materials

The epoxy resin was a bifunctional diglycidyl ether of bisphenol-A
(YD-128 from Kuk Do Chem., Seoul, Korea), and the curing agent was
4,4'-methylene dianiline (Kuk Do Chem.). The epoxy equivalent
weight of the epoxy resin was 187 g/mol, and the viscosity of the resin
was about 12,000 cP at 25[degrees]C. The organophilic smectite clay
(Cloisite 30B, a chemically modified montmorillonite) was supplied by
Southern Clay Products, USA. The organic modifier used to modify
pristine clay ([Na.sup.+] montmorillonite) was methyl tallow bis-2-hydroxyethyl quaternary ammonium. Tallow means, predominantly, an
octadecyl chain with smaller amounts of lower homologues (approximate
composition: ~65% CI8; ~30% CI6; ~5% C14). It is noteworthy that the
organic modifier has two hydroxy 1 groups, which may affect the curing
behavior of the epoxy resin. The modifier concentration in the
organoclay was 90 mequiv/100 g clay, and the interlayer spacing of
silicate layers was 1.86 nm. Figure 1 shows the structures of the
materials used in this study.

Preparation of Epoxy/Clay Nanocomposites

A certain amount (1, 3, and 5 phr (parts per hundred of epoxy
resin)) of the organoclay was mixed with the epoxy resin, and stirred
using a mechanical stirrer at room temperature. The mixing time was
changed up to 12 h. to see the effect of mixing time on the structure
and properties of the epoxy/clay nanocomposite system. And then, the
binary mixture of the epoxy resin and the organoclay was mixed again
with the curing agent by stoichiometry, and stirred for 5 min. After
degassing the ternary mixture in a vacuum oven at 60[degrees]C for 5 min
to make an epoxy/clay nanocomposite sample, for mechanical tests and
structure analyses, it was poured into a silicon rubber mold (35 mm x 13
mm X3.2 mm) and then cured in a hot press at 170[degrees]C for 2 h, and
finally postcured at 200[degrees]C for 30 min.

Instruments

Differential Scanning Calorimetry. In order to investigate the
curing behavior of the epoxy/clay nanocomposite system, the DSC 2910 (TA
Instruments, New Castle, DE) was used. About 10 mg of the degassed
ternary mixture before curing was placed in a hermetic aluminum liquid
sample pan, and the sample pan was tested immediately after sealing and
positioning it right on the differential scanning calorimetry (DSC)
sample cell. Each sample was cured dynamically at different scanning
rates of 5, 10, and 20[degrees]C/min, respectively. The dynamic DSC
scanning temperature range was from 10 to 200[degrees]C under a nitrogen
gas flow (65 ml/min). Dynamic DSC second scans were performed at a
scanning rate of 10[degrees]C/min, to investigate the glass transition
behavior of the nanocomposites.

Dynamic Mechanical Analysis. Thermomechanical properties of the
fully cured epoxy/clay nanocomposites were investigated using the DMA 2940 (TA Instruments) mounted with a single cantilever. The frequency
was 1 Hz, and the scanning rate was 5[degrees]C/min. The scanning
temperature range was from room temperature to 300[degrees]C.

X-ray Diffractometer. X-ray diffractometer (XRD; SCINTAG XDS 2000,
Scintag. Cupertino, CA) was used to analyze the nanostructures of the
epoxy/clay nanocomposites. The XRD instrument was equipped with Cu
K[alpha] radiation (wavelength = 0.15418 nm). The scanning was carried
out from 1.5[degrees] to 10[degrees] at a rate of 0.6[degrees]/min.

Transmission Electron Microscope. Transmission electron microscope
(TEM) photographs of the cured epoxy/clay nanocomposites were obtained
using the JEM-2020 (JEOL, Tokyo, Japan). To make specimens for TEM
analyses, the epoxy/clay nanocomposites were microtomed by Leica
Ultracut-R into about 80-nm thick slices. A carbon layer was deposited
on these slices, followed by placing them on a 400-mesh copper grid for
TEM imaging.

RESULTS AND DISCUSSION

Reaction Kinetics

The curing reaction of a thermoset resin system results in an
increase of molecular weight with curing lime. The polymerization process of the thermoset resin system can be characterized by gelation and vitrification. The attainment of a gel formation threshold
corresponds to the formation of a macroscopic crosslinked structure that
constrains the mobility of chains, supposing an increasing loss of
fluidity until viscosity rises toward an infinite value. The
vitrification usually appears when the glass transition temperature of
the reacting medium exceeds the reaction temperature of the system
during polymerization. So the vitrification can be observed during a
step-by-step isothermal polymerization procedure, or during a dynamic
polymerization at a very low heating rate. But the curing reaction,
which has been stopped temporarily by vitrification, can be resumed upon
subsequent heating of the reacting medium above its glass transition
temperature.

There have been several ways to investigate the curing kinetics of
a thermoset resin system. One of them is to measure the change of a
specific physical property that can be related to the chemical
conversion during polymerization. These properties include rheological
properties, electrical properties, thermomechanical properties, and heat
evolution by reaction. Among these methods, the thermal analysis by DSC
[21, 22], measuring heat evolution by reaction, has an advantage of
simultaneously providing the kinetic and thermal data on the resin
system.

The epoxy curing reactions by amines are exothermic and analyzed by
somewhat different order kinetics, because they indicate different
curing characteristics to each other. Shechter et al. [23] investigated
the chemistry about the curing of bifunctional epoxy resins by amine hardeners and found that a combination of an epoxide and a primary amine leads to two principal reactions; (1) the addition reaction of a primary
amine hydrogen to an epoxy group to form a secondary amine and (2) the
addition reaction of an amine hydrogen in the secondary amine formed by
the reaction (1) to another epoxy group to create a tertiary amine.
These epoxy curing reactions by amines are known to be autocatalytic,
because the OH groups formed during the reaction helps in the ring
opening of epoxy groups [24].

A simple n-th order reaction kinetic model can be expressed as
follows:

d[alpha]/dt = k(1 - [alpha])[.sup.n] (1)

where k is the reaction rate constant and n is the reaction order.
This simple n-th order reaction kinetics is generally used for a
polyurethane system. This model assumes a maximum initial reaction rate
and, consequently, is not capable of describing an epoxy curing reaction
by an amine hardener, which exhibits a maximum reaction rate during
isothermal curing because of the autocatalytic effect by OH groups.
Kamal and Sourour [25] proposed the following semi-empirical reaction
kinetic model that could successfully describe the autocatalytic
reaction mechanism of an epoxy curing reaction by an amine hardener,

The reaction rate constants, [k.sub.1] and [k.sub.2], are usually
assumed to have an Arrhenius temperature dependence. This autocatalytic
kinetic equation has been found to describe well the epoxy curing
reactions by amine hardeners.

Mechanistic models are obtained from the balances of reacting
species. Thus, a good understanding of a reaction mechanism is required.
Therefore, mechanistic models are better than the phenomenological ones
in terms of prediction and interpretation of curing reaction kinetics of
a thermosetting system. However, because thermosetting reactions are
rather complex, mechanistic models are not always feasible. Therefore,
in this study, the phenomenological model expressed as Eq. 2 was used to
analyze the reaction kinetics of the epoxy/clay nanocomposite system.
The overall reaction order was assumed to be 2, in this study, not only
to make the kinetic model have some mechanistic aspect but also because
this assumption was reasonable for other thermosetting resin systems
similar to the system of this work [26]. The rate constants, [k.sub.1]
and [k.sub.2], can be described as follows, using an Arrhenius
temperature dependence:

[k.sub.1] = [k.sub.11] exp([-E.sub.1]/RT) (3)

[k.sub.2] = [k.sub.22] exp([-E.sub.2]/RT) (4)

where [k.sub.11] and [k.sub.22] are frequency factors, [E.sub.1]
and [E.sub.2] are activation energies, and R is the ideal gas constant.

The following autocatalytic reaction kinetic equation was obtained
by combining the aforementioned equations, Eq. 2-4, and introducing the
scanning rate term ([S.sub.r]) to analyze directly the reaction kinetic
data obtained from the dynamic DSC experiments.

The dynamic DSC experimental technique was used to obtain reaction
kinetic data of the epoxy/clay nanocomposite system. Figure 2a shows the
dynamic DSC thermograms of the pure epoxy resin system for various
scanning rates. The peak temperature showing a maximum heat evolution
shifted to a higher temperature region with increasing scanning rate.
This peak-shifting phenomenon caused by increasing the scanning rate
depends on the activation energy associated with each reaction. Based on
this peak-shifting phenomenon, there have been two methods discussed in
the literature to calculate the activation energy associated with each
reaction. They are Kissinger's method [27] and the method suggested
by Ozawa [28] and Flynn [29]. But these methods are not sufficient in
analyzing the reaction kinetics of a thermosetting system accurately,
for a whole range of the reaction, because they use only limited
information from the DSC thermograms. So a numerical fitting method,
which uses all the experimental reaction kinetic data calculated from
the dynamic DSC thermograms for a whole range of the reaction, was used
to analyze the reaction kinetics of the epoxy/clay nanocomposite system
in this study. To obtain conversion data from the dynamic DSC
thermograms, the conversion was assumed to be the ratio of the reaction
heat generated until a certain temperature, HT, to the overall heat of
reaction at complete conversion, [H.sub.rxn]. The overall heat of
reaction. [H.sub.rxn], for each epoxy/clay nanocomposite system was
obtained by integrating each dynamic DSC thermogram, respectively.

[FIGURE 2 OMITTED]

The reaction kinetic parameters of the kinetic equation were
determined by fitting the dynamic DSC conversion data to the kinetic
equation, Eq. 5, using the Marquardt's multivariable nonlinear
regression method and Runge-Kutta integration technique [30]. Three sets
of experimental conversion data obtained from the three dynamic DSC
thermograms, respectively, were fitted simultaneously by the kinetic
equation, to determine accurate and reasonable values of the reaction
kinetic parameters. The values of the reaction kinetic parameters,
[k.sub.1], [k.sub.2], [E.sub.1], [E.sub.2], and n, determined by the
fitting process are listed in Table 1 for each epoxy/clay nanocomposite
system together with the pure epoxy resin system. Figure 2b shows that
the conversion data obtained from the dynamic DSC thermograms agree well
with the conversion curves calculated from the reaction kinetic equation
for the pure epoxy resin system. The second order autocatalytic reaction
kinetics could describe well the reaction kinetics of the pure epoxy
resin system.

Figure 3a shows the theromograms of the epoxy/clay nanocomposite
system for various clay contents obtained at a scanning rate of
10[degrees]C/min. The peak temperature shifted to a lower temperature
region with increasing clay content: 169.0[degrees]C for 0 phr,
166.7[degrees]C for 1 phr, 165.6[degrees]C for 3 phr. and
164.5[degrees]C for 5 phr. This result shows that the curing rate of the
nanocomposite system increased as the clay content was increased. This
increase in curing rate with increasing clay content was considered to
be due to the OH groups of the organic modifier of the clay, which could
accelerate the epoxy curing reaction. Figure 3b shows that the
conversion data obtained from the dynamic DSC thermograms agree well
with the conversion curves calculated from the aforementioned reaction
kinetic equation for the epoxy/clay nanocomposite system. The second
order autocatalytic reaction kinetics could describe well the reaction
kinetics of not only the pure epoxy resin system but also the epoxy/clay
nanocomposite system.

[FIGURE 3 OMITTED]

Dynamic Mechanical Properties

Dynamic mechanical analysis (DMA), which measures the modulus and
energy dissipation properties of a material, was carried out to
determine both the glass transition temperatures and thermomechanical
properties of the cured epoxy/clay nanocomposites. Two different moduli
of the nanocomposites, a storage modulus (E') which is related to
the ability of the material to return or store energy, and a loss
modulus (E") which is related to the ability of the material to
dissipate energy, were determined as a function of temperature. The
temperature dependence of the ratio, E'/E", which is called
tan delta (tan 8) of the material, was also determined as a function of
temperature.

Figure 4a shows the dependence of the storage modulus of the
epoxy/clay nanocomposites containing various clay contents on
temperature. Compared to the pure epoxy resin system, the storage moduli
of the nanocomposites were very slightly increased. The glass transition
temperature of the epoxy/clay nanocomposite system, which can be
determined by taking the temperature of the most drastic decrease in the
storage modulus, increased with increasing clay content. The increase in
the glass transition temperature of the nanocomposite system was
considered to be due to the dispersion of the silicate layers of the
clay and their ability to hinder the motion of the molecular chains and
network junctions.

Figure 4b shows the dependence of the storage modulus of the
epoxy/clay nanocomposite containing the clay 3 phr on temperature as
well as the mixing time during the nanocomposite sample preparation
step. With increasing mixing time, the storage modulus and the glass
transition temperature of the epoxy/clay nanocomposite increased first,
and then almost unchanged after that, because a sufficient mixing was
already attained. The initial increase with the mixing time in the
storage modulus and the glass transition temperature of the
nanocomposite was considered to be due to the structure change of the
nanocomposite with the mixing time. The nanocomposite would have more
exfoliated and more intercalated clay structure with increasing mixing
time, and this structure change would result in the increase in the
storage modulus and the glass transition temperature. The structure
change with the mixing time was investigated by XRD and TEM, and the
results are shown in the following section. The mixing time of 12 h was
considered to be sufficient for the epoxy/clay nanocomposite system,
because a mixing time more than 12 h showed a negligible effect on the
thermomechanical properties of the nanocomposites. The DMA data shown in
Fig. 4a are for the epoxy/clay nanocomposites prepared by the mixing
time of 12 h.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Structure and Morphology

The XRD patterns of the clay and the epoxy/clay nanocomposites
containing different amounts of clay are shown in Fig. 5a. The peaks for
the epoxy/clay nanocomposites were observed at a lower diffraction angle
region compared to the peak for the clay, and their positions were
almost same regardless of clay content. The peak shift to a lower angle
region means that the interlayer spacing of the clay was increased by
the intercalation of organic molecules between the silicate layers of
the clay. But the fact that the peak positions of the nanocomposites
were negligibly affected by clay content means that the interlayer
spacing was determined predominantly by chemical and physical
interactions between the clay and the epoxy resin system. This
consideration could also be supported by the XRD patterns shown in Fig.
5b, demonstrating the effect of mixing time on the structure of the
nanocomposite containing the clay 3 phr. With the increase in the mixing
time the peak height was decreased, because the portion of the clay
exfoliated into the epoxy matrix, among the total amount (3 phr) of the
clay incorporated in the epoxy resin system, would be increased
gradually with the increase in the mixing time. Just a negligible XRD
peak shift to a lower angle region was observed with increasing the
mixing time, even though the peak height was considerably decreased. By
the natural chemical and physical interactions between the clay and the
epoxy resin system, the intercalation process was found to progress
quite rapidly, because the XRD-peak shift to a lower angle region due to
the intercalation of the epoxy resin system was almost completed in 10
min, as shown in Fig. 5b.

Using the Bragg equation, 2d sin[theta] = n[lambda], where d
denotes the lattice spacing of the clay, 2[theta] the angle of the XRD
peak, n a positive number (generally 1), and [lambda] is the wavelength
(0.15418 nm) of the X-ray, the interlayer spacing of the silicate layers
of the clay could be calculated. The interlayer spacing of the silicate
layers of the clay was calculated to be 1.86 nm from the XRD peak
position of the clay (2[theta] = 4.75[degrees]). The interlayer spacing
of the silicate layers of the clay in the nanocomposites was calculated
to be about 3.68 nm from the XRD peak positions (20 = 2.40[degrees]) of
the nanocomposites, as shown in Fig. 5a. The interlayer spacing of the
silicate layers of the clay in the nanocomposites was increased by two,
by the intercalation of the epoxy resin system compared to the clay
itself. The interlayer spacing of the silicate layers of the clay in the
nanocomposite containing the clay 3 phr was 3.53 and 3.68 nm according
to the XRD peak positions (2[theta] = 2.50[degrees] and 2.40[degrees],
respectively) of the nanocomposite for two different mixing times of 10
min and 12 h. respectively. The increase in the interlayer spacing of
the silicate layers of the clay was negligible with increasing mixing
time, because the intercalation process progressed rapidly by the
natural chemical and physical interactions between the clay and the
epoxy resin system. Figure 5b indicated that with increasing the mixing
time, the silicate layers of the clay were readily intercalated at the
early stage of mixing, and then the exfoliation of the silicate layers
of the clay progressed gradually.

Although the XRD patterns gave some information on the interlayer
spacing of the silicate layers of the clay in the epoxy/clay
nanocomposites, information on the morphology and the exfoliation
structure of the nanocomposites is not sufficient. Therefore. TEM was
also used to visually evaluate the degree of intercalation and
exfoliation of the clay, the amount of aggregation of clay clusters, and
the morphology of the nanocomposites. TEM photographs of the epoxy/clay
nanocomposite containing the clay 3 phr are shown in Fig. 6 for two
different mixing times. Compared to the TEM image for the mixing time of
10 min, the TEM image for the mixing time of 12 h showed a little bit
more intercalated and exfoliated structure. The findings from the TEM
photographs agreed well with the findings from the XRD patterns.

[FIGURE 6 OMITTED]

CONCLUSIONS

The curing behavior, thermomechanical properties, and structures of
the epoxy/clay nanocomposite system were studied in this work. The
curing rate of the epoxy/clay nanocomposite system increased slightly
with increasing clay content. The reaction kinetic parameters of the
kinetic equation were determined by fitting the dynamic DSC conversion
data to the kinetic equation, using the Marquardt's multivariable
nonlinear regression and Runge-Kutta integration techniques. The fitting
results showed that the reaction kinetics of the nanocomposite system
could be described well by the autocatalytic second order reaction
kinetic equation. The glass transition temperature of the epoxy/clay
nanocomposite system was higher than the pure epoxy resin system, and
increased slightly with increasing clay content. The storage modulus of
the epoxy/clay nanocomposite system was slightly higher than the pure
epoxy resin system, and increased slightly with increasing clay content.
The XRD patterns and TEM photographs indicated the formation of dominant
intercalated structures in the epoxy/clay nanocomposites together with
some exfoliated structures.